The present invention relates to a process for removing impurities from gas streams utilizing ozone, ultraviolet radiation, or a combination of both. More particularly, the present invention provides for means to remove simultaneously multi-pollutants such as nitrogen oxides, sulfur oxides, mercury and mercuric oxide from gas streams containing these impurities.
Recent federal and local environmental laws require very significant reduction of discharge of harmful gaseous substances into the atmosphere. Chief among such harmful air pollutants are nitrogen oxides (NOx). In response to strict enforcement efforts of these laws, industrial air polluters have made considerable efforts to reduce the amount of these harmful substances into the air in gaseous effluents from industrial or municipal sources. Successful efforts to reduce the concentration of NOx in gaseous effluents often involve reacting the NOx in waste gases with nitrogen-based reducing agents.
Another known method of removing NOx from gas streams involves contacting the NOx with ozone, thereby oxidizing them to higher nitrogen oxides, such as N2O5 and removing the higher oxides from the gas stream by means of aqueous scrubbers.
Specific details of ozone-based NOx oxidation processes are disclosed in U.S. Pat. Nos. 5,206,002; 5,316,737; 5,985,223; and 6,197,268, the disclosures of which are incorporated herein by reference.
Additionally, the Environmental Protection Agency has presented information showing mercury levels in the environment are at levels that are likely to lead to adverse health effects. Coal-fired utility boilers are one of the largest sources of harmful anthropogenic mercury emissions but also include the NOx and SOx emissions sources. The present inventors have discovered a process whereby mercury as well as NOx and SOx can be removed from the emission from coal-fired utility boiler flue gas.
The present invention provides for a process for removing impurities such as nitrogen oxides, sulfur oxides and mercury from a gas stream comprising the steps of passing the gas stream into a reaction zone or duct, reacting the impurities with ozone to form N2O5 and mercuric oxide and passing the gas stream containing these oxides to an aqueous alkaline scrubber to remove the N2O5 and mercuric oxide, as well as the sulfur oxides present in the gas stream.
The present invention also provides for a process for removing nitrogen oxides, sulfur oxides and mercury from a gas stream comprising the steps of passing the gas through a wet scrubber, then passing the gas, which is now free of water-soluble acid gases and particulate matter, to a reaction duct where ozone is inputted to oxidize nitrogen oxides and mercury. The gas stream is then passed to a second wet scrubber containing an acidic medium where soluble higher orders of nitrogen oxides and metallic oxides are removed. Lastly, the gas stream is directed to a third wet scrubber system where sulfur oxides are removed, resulting in a cleaner gas stream for emission to the atmosphere.
Alternatively, the present invention provides for a process for removing nitrogen oxides and mercury from a gas stream whereby the gas stream is directed to a reaction duct where ozone is inputted. The resulting gas stream is then passed to a wet acid scrubber where particulate matter, and higher order of nitrogen oxides and mercuric oxide, formed in the reaction duct, are removed from the gas stream. This gas stream is then directed to a chamber whereby the gas stream is irradiated for a sufficient time by ultraviolet radiation which causes the remaining mercury to react with the oxygen. The resulting gas stream is then passed to an aqueous scrubber whereby the mercuric oxide is washed out of the gas stream.
The present invention also provides for a process for removing impurities from a gas stream which may include mercuric oxide as well as nitrogen oxides, sulfur oxides and elemental mercury. The gas stream is directed to an aqueous alkaline scrubber where sulfur oxides, mercuric oxide, when present, acid gases and particulates are removed. This gas stream passes to a reaction zone or duct where the nitrogen oxides and mercury present in the gas stream will react with ozone to form N2O5 and mercuric oxide. The gas stream containing these impurities is then directed to another aqueous alkaline scrubber where the oxides and mercuric oxide be scrubbed out from the gas stream.